1. Field of the Invention
The present invention relates to an electrophoretic display device and a method of driving the same and more particularly to the electrophoretic display device capable of providing excellent displaying by preventing an afterimage and/or image burn-in and to the method of driving the electrophoretic display device.
The present application claims priorities of Japanese Patent Application Nos. 2005-362318 filed on Dec. 15, 2005 and 2005-378274 filed on Dec. 28, 2005, which are hereby incorporated by reference.
2. Description of the Related Art
One example of electronic displays which enable reading of an electronic book, electronic newspaper, or a like with human eyes without causing stress on the eyes is an electronic paper display which is being developed earnestly. Requirements for the electronic paper display are to be thin, lightweight, resistant to breaking (cracking), easy to see at a printed level or, a like. As a display device that can satisfy these requirements, a reflective-type display is available which is so configured as not to use a backlight and to consume less power.
An example of the reflective-type display using no polarizer includes an electrophoretic display (hereafter called “EPD”) or a like. There are several types of EPDs and an EPD using a microcapsule-type electrophoretic device (also referred simply as an “electrophoretic element”) is described below.
FIG. 25 is an enlarged cross-sectional view conceptually showing configurations of an electrophoretic display panel and more particularly a cross-sectional view of monochrome microcapsule-type electrophoretic elements arranged in m-rows and n-columns in a matrix form. In the electrophoretic display panel, each of the microcapsule-type elements, as shown in FIG. 25, is formed in a layer-stacked structure in which a TFT (Thin Film Transistor) glass substrate 102, an electrophoretic film 110, PET (Polyethylene Terephthalate) facing substrate 120 are stacked in this order, all serving to enable active-matrix driving of the electrophoretic display device, and, for example, microcapsule-type electrophoretic elements 100-m1, 100-m2, and 100-m3 are formed in the m-rows.
On the TFT glass substrate 102 are formed TFT 104-m1, TFT 104-m2, and TFT 104-m3 each corresponding to each of the electrophoretic elements 100-m1, 100-m2, and 100-m3, pixel electrodes 106-m1, 106-m2, and 106-m3 each being connected to each of the TFT 104-m1, 104-m2, and 104-m3, and storage electrodes 108-m1, 108-m2, and 108-m3 each being formed in a manner to face each of the pixel electrodes 106-m1, 106-m2, and 106-m3. Thus, the microcapsule-type electrophoretic display device is constructed to display images by an active-matrix driving method. In a binder 112 made of polymer housed in the electrophoretic film 110, microcapsules being about 40 μm in size are spread all over within the binder 110. Conventionally, each of the microcapsules 114 is smaller by a specified value than a dimension of the pixel electrode of the microcapsule-type electrophoretic display device. Into each of the microcapsules 114 is injected a dispersant 116 in which a myriad of negatively-charged white pigment particles (white particles, for example, titanium oxide) 117 with the size at a nano-level and positively-charged black pigment particles (black particles, for example, carbon) 118 with the size also at the nano-level are suspended. In the PET facing substrate 120, one piece of a counter electrode 122 which faces the pixel electrodes 106-m1, 106-m2, and 106-m3 formed on the TFT glass substrate 102 is stuck to a plastic substrate 124. Therefore, each of the microcapsule-type electrophoretic elements 100-m1, 100-m2, and 100-m3 is made up of each of the TFT 104-m1, 104-m2, and 104-m3 corresponding to each of the pixel electrodes 106-m1, 106-m2, and 106-m3, the microcapsules 114, and a corresponding portion of the counter electrode 122.
FIG. 26 is a schematic circuit diagram of the microcapsule-type electrophoretic elements arranged in a matrix and in a plane form, which makes up the microcapsule-type electrophoretic display device (hereafter simply an “electrophoretic display device”). In FIG. 26, same reference numbers are assigned to components having the same function as in FIG. 25. In FIG. 26, a data line Dn typifies lines used to feed display data signals to each of the electro-phoretic elements 100-mi (i=1, 2, . . . , N) arranged in a horizontal direction, out of the electrophoretic elements 100-mn (m=1, 2, . . . , M, n=1, 2, . . . , N) arranged in a matrix form, which make up the electrophoretic display device. Moreover, a scanning line Gm typifies lines used to feed scanning voltages, during one scanning period, to the electrophoretic elements 100-m1, 100-m2, . . . , 100-mN arranged in a horizontal direction, out of the electrophoretic elements 100-mn arranged in a matrix form, which also make up the electrophoretic display device.
FIG. 27 is a schematic circuit diagram showing a driving circuit 140 of the conventional electrophoretic display device. The driving circuit 140 includes a scanning driver 142 to sequentially feed scanning voltages during one scanning period to each electrophoretic element group (100-m1, 100-m2, . . . , 100-mN) arranged in the horizontal direction, out of the electrophoretic elements arranged in the matrix form and a data driver 144 to sequentially feed display data signals through each data line Dn to each of the electrophoretic elements 100-mi arranged in the horizontal direction, out of the electrophoretic elements arranged in the matrix form. FIG. 28 is a schematic circuit diagram showing a data signal generating circuit 145 for every data line Dn, which makes up the data driver 144. The data signal generating circuit 145 includes a selecting signal generating circuit 146 to generate a selecting signal in response to picture data and a voltage selecting circuit 147 to output a voltage corresponding to a selecting signal output from the selecting signal generating circuit 146 to the data line Dn.
In the electrophoretic display device having configurations described above, a voltage is applied, in such a way as described below, to the pixel electrodes 106-mn making up the microcapsule-type electrophoretic elements 100-mn and an image corresponding to picture data input to the picture of the electrophoretic display device is displayed on its picture.
When the electrophoretic element 100-mn corresponding to a pixel on the picture of the electrophoretic display device is made to serve as a unit of displaying a white state (hereinafter simply “W”), a negative voltage is output to the pixel electrode 106-mn making up the electrophoretic elements 100-mn; that is, for example, a voltage of −15V is output from the data driver 144 to a data line, for example, to the data line Dn of the data driver 144 connected to the pixel electrode 106-mn during a period corresponding to required numbers of frames. This operation being described by referring to FIG. 28, the selecting signal generating circuit 146 receiving picture data outputs the negative voltage to a selecting line corresponding to the above pixel, for example, the selecting line 152-n during a period when the pixel is operating. This causes a pMOS (p-channel Metal Oxide Semiconductor) transistor, for example, the pMOS 154-n making up the voltage selecting circuit 147 to be turned ON and the voltage of −15V to be output to the data line Dn.
Also, when the electrophoretic element corresponding to a pixel on the picture of the electrophoretic display device is made to serve as a unit of displaying black (hereinafter simply “B”), a positive voltage is output to the pixel electrode 106-mn making up the electrophoretic element; that is, for example, a voltage of +15V is output from the data driver 144 to a data line, for example, to the data line Dn of the data driver 144 connected to the pixel electrode 106-mn during a period when required numbers of frames are displayed. This operation being described by referring to FIG. 28, the selecting signal generating circuit 146 receiving picture data outputs the negative voltage to the selecting line corresponding to the above pixel, for example, the selecting line 156-n during a period when the pixel is operating. This causes a pMOS (p-channel Metal Oxide Semiconductor) transistor, for example, the pMOS 158-n making up the voltage selecting circuit 147 to be turned ON and the voltage of +15V to be output to the data line Dn.
In the electrophoretic display device to display images in monochrome, owing to the memory characteristic that its electrophoretic element has, when display of a pixel is switched from W to B or from B to W, the application of such a voltage as described above to a pixel electrode of the electrophoretic element 100-mn corresponding to the pixel whose display is to be switched, however, when display of a pixel is switched from W to W, and from B to B, basically, the application of the voltage to the pixel is not required.
Next, driving of such an electrophoretic display device analyzed by the inventor of the present invention is described below. As described above, in the electrophoretic film 110, when display of a pixel is changed from W to B, it is necessary to apply a positive voltage to a pixel electrode and when display of a pixel is changed from B to W, it is necessary to apply a negative voltage to the pixel electrode, and when display of a pixel is changed from W to W, and from B to B, it is necessary to apply a voltage of 0V.
Moreover, in the case of an active-matrix type display device such as a liquid crystal display device, a picture can be rewritten during a period corresponding to one frame being 1/60 Hz (=16.6 ms). However, in the case of the electrophoretic display device, it is impossible to rewrite a picture during a period corresponding to one frame being 1/60 Hz (=16.6 ms). The reason for this is that, for example, in the microcapsule-type electrophoretic element making up the electrophoretic display device, the particles 117, 118 are sealed in the microcapsules 114 filled with a dispersant and the particles 117, 118 therein have a slow response and, as a result, rewriting of a picture cannot be completed unless a voltage continues to be applied during a period while a plurality of frames is displayed. Therefore, in the electrophoretic display device, generally, as shown in FIG. 29, a PWM (Pulse Width Modulation) driving method is employed in which, when display of a pixel is changed from B to W, a specified negative voltage continues to be applied during a period corresponding to a plurality of frames and, when display of a pixel is changed from W to B, a specified positive voltage continues to be applied during a period corresponding to the plurality of frames.
In the conventional electrophoretic display device, in order to achieve the driving method as shown in FIG. 29, it is supposed that, a difference is calculated between a voltage applied to a current picture stored in a frame buffer made up of SRAMs (Static Random Access Memories) and a voltage applied to its next picture and, when display of a pixel is changed from B to W and from W to B, a corresponding voltage is applied, based on the calculated difference in the voltages, during a period corresponding to a plurality of frames. To apply these voltages, a ternary (+V, 0V, and −V) driver is used as a H-driver and Vcom is set to be 0V. Changing of display on a picture from B to W and from W to B is made at time when the corresponding frames are displayed.
However, further analysis by the inventor has demonstrated that the conventional electrophoretic display device described above has technological problems. That is, when the conventional microcapsule-type electrophoretic element is driven in the driving way described in FIG. 30, a decrease of white luminance or an increase of black luminance was found when the microcapsule-type electrophoretic element with a voltage being not applied to its pixel electrode is driven, not only due to a memory characteristic of its microcapsule-type electrophoretic element but also due to influences by a gate line and/or data line of the microcapsule-type element or to DC (Direct Current) component contained in common potentials of a counter electrode. As a result, when display is switched from W to W and from B to W, a difference in white luminance occurs (see FIG. 31 and FIG. 32) and the first afterimage problem arises that, while the next picture is being displayed, a current picture remains persistent. Also, the same problem arises when display of a picture is changed from B to B and from W to B.
Also, when a high-definition electronic book display terminal device is fabricated, when a dither pattern is displayed in two gray levels, or when images are made to be colored, it is necessary to set a pixel pitch to be 150 μm or less. However, it was found that, if a pixel pitch was made narrower, a microcapsule-type electrophoretic element was affected by a pixel voltage applied to a neighboring microcapsule electrophoretic element. More specifically, it was also found that, in order to display a dither pattern in two gray levels, when a pattern in a current image is displayed in black and a pattern in a next image is displayed in a checkered manner, a black display region on a picture is damage, that is, a display region originally prepared for pixels is reduced. When a black character of some regional type is displayed on the current picture and a dither pattern is displayed on the next picture, the second afterimage problem occurs that the character displayed on the current picture remains persistent on the next picture.
The above problem was found to occur due to the reason that, according to the conventional driving method, since no pixel voltage is applied to the pixel for a character of “NTL” displayed in black on the current picture shown in FIG. 33 and to the pixel for the dither pattern displayed in black on the next picture, in the case where a pixel electrode is a fine and small pattern having the size of 100 μm to 150 μm, the pixel with no voltage applied picks up a voltage applied to a neighboring pixel for white display and, as a result, white particles appear toward a surface of the microcapsule placed on the pixel electrode of the neighboring pixel (see FIG. 34).
As described above, when display on a picture is changed sequentially, for example, from B to W, from W to B, and from B to W and positive or negative voltages of +15V, −15V, +15, and −15V are applied alternately to pixel electrodes of pixels and, therefore, no DC current is applied to the electrophoretic element. However, if display on the picture is changed continuously from B to B, then from B to B, and further from B to B, and a voltage of +15V is applied to pixel electrodes during a period corresponding to many frames, or if display on the picture is changed from W to W, then from W to W, and further from W to W and a voltage of −15V continues to be applied to pixel electrodes during a period corresponding to many frames and, as a result, a positive or negative DC potential continues to be constantly applied to the electrophoretic elements to which the above +15V or −15V is applied. Therefore, it was found that a charged-up damage occurs in the electrophoretic film and, even if display of images are terminated by applying 0V, inverted image that displayed only the charged-up portion is still displayed, causing an image burn-in.